This invention discloses certain improvements to the invention disclosed and claimed in U.S. Pat. No. 5,315,527 issued to Robert W. Beckwith on May 24, 1994 (also the inventor of the present invention) entitled "METHOD AND APPARATUS PROVIDING HALF-CYCLE DIGITIZATION OF A-C SIGNALS BY AN ANALOG-TO-DIGITAL CONVERTER"; said patent being incorporated herein by reference.
U.S. Pat. No. 5,315,527 discloses the digitization of portions of an alternating current (AC) signal of selected polarity for use where all of the information necessary to make desired measurements of the entire signal is contained in a selected portion of the signal. U.S. Pat. No. 5,315,527 discloses certain inventive advantages in the use of a high sampling rate in measuring the amplitude of an AC signal using a charge coupled analog to digital converter (ADC). U.S. Pat. No. 5,315,527 further describes inventive ways of measuring the amplitude of at least two AC signals and the phase angle between them. The measurement of the average and the RMS value of amplitude are disclosed, as well as the measurement of phase angle as the time between zero crossings of the AC signals.
U.S. Pat. No. 5,224,011 issued to Murty V. V. S. Yalla et al. discloses an all-digital device for electric power system protective relaying. In U.S. Pat. No. 5,224,011, control voltage or current signals are sampled by an ADC and the samples processed using Fourier Transforms in order to extract factors required for computation essential to performance of said device. These samples are taken at a rate established by a multi-task operating system used by an associated microprocessor.
U.S. Pat. No. 4,419,619 issued to James A. Jindrick et al., dated Dec. 6, 1983, (Jindrick) describes the use of microprocessor and software modules for the control of a voltage regulating transformer. Jindrick requires transforming digital voltage signals from the time domain into the frequency domain to obtain a measured digital voltage signal representative of the RMS voltage. In contrast to Jindrick, the present invention computes the RMS amplitude of a signal directly in the time domain and does not transform into the frequency domain. The present invention also provides for the alternate computation of the average amplitude of an input AC analog signal, or the computation of the peak value of an input analog signal, all being computed directly in the time domain.
Furthermore, Jindrick requires processing a number of cycles to obtain the RMS value whereas the present invention develops its full measurement resolution on each half cycle sampled. Jindrick requires the use of a software operating system whereas the inventive program requires essentially no operating system. The MERTOS operating system referenced by Jindrick may in itself be as large as the entire program required by the present invention.
Another prior art signal sampling method is to synchronize a specific number of samples with the power frequency thus fixing the number of samples per cycle regardless of slow changes in the power frequency. Such a method is described in an article by Gabriel Benmoual entitled "AN ADAPTIVE SAMPLING-INTERVAL GENERATOR FOR DIGITAL RELAYING" published in IEEE Transactions on Power Delivery, Vol. 4 No. 3 July 1989. In the present invention and in contrast to Benmoual, the signal sampling rate is synchronized with a free running ADC in order to more effectively utilize the sampling rate capability of said ADC.
Most of the foregoing methods require an analog anti-aliasing filter in each AC signal channel ahead of the ADC for the purpose of avoiding alaising or the incorrect assimilation or misinterpretation of harmonic frequency components of said AC signal. The high sampling rate of the present invention eliminates the necessity for anti-alaising filters and, furthermore, improves the amplitude measurement accuracy of a single AC signal cycle as will be described hereinafter.
Furthermore, much of the hardware, software and operating time in equipment typified by the patents and articles listed above is used in data processing and communications resulting in added cost and reduced efficiency in the fundamental task of control or protection.
The present invention provides alternative communications having higher data rates than usable over a telephone circuit, and passes the data processing and communications work to a personal computer that is only required when such data processing and communications are desired. Only a very few percent of the inventive hardware, software and operating time is used in serving this external computer.
Furthermore, controls and protective relays, as described above, devote much hardware, software, and operating time in providing an operator interface which may often be used for only brief periods in a years time. Inventive ways of using said personal computer (PC) for the operator interface are described hereinunder whereby said PC need only be connected when required as an interface.